Much effort has recently been directed to developing safety systems which assist vehicle drivers by detecting objects located in the so-called blind spot of the driver's vision or field of view. The blind spot exists due to the limited scope of view provided by conventional systems of rear and side view mirrors. The existence of the blind spot is well known and requires a practice known as the shoulder check prior to, for example, changing lanes. In a shoulder check, the driver must physically turn around and momentarily look backward to see if the way is clear for the vehicle to move sideways to change lanes. Often this is done under pressure and is subject to frequent misjudgments and often enough a shoulder check fails to reveal a vehicle in the blind spot. The problem is compounded in heavy traffic where numerous shoulder checks may be required before being able to find an opening to change lanes. In heavy traffic the likelihood of a sudden speed change of the traffic ahead is also increased so having the driver's attention diverted from the front to make shoulder checks is dangerous. What is needed is a device to supplement shoulder-checks to give a greater certainty that the blind spot is not occupied.
My own prior patents have described inventions to assist drivers in determining whether an object, such as another vehicle, is present in the blind spot by means of remote sensing technology. In particular in U.S. Pat. No. 6,753,766 there is disclosed a detecting device and method of using the same for this very purpose. This prior patent teaches using at least a first detector mounted to the moving host vehicle and aligned to a stationary target area for generating at least a first output representative of the stationary target area at a time T1 and at least a second detector mounted to the host vehicle and aligned to the stationary target area for generating a second output representative of the stationary target area at least at a second time T2. A control system receives said first and second outputs and compares the first output to the second output to identify differences between the outputs. If there are enough differences between the signals, i.e. above a noise threshold, then this means that something is moving in the stationary target area and an alarm is given. If the two signals are the same, or within a noise threshold, that means there has been no change to the stationary target area between time T1 And T2 and so the small difference between the signals represents a stationary target. In this manner a target vehicle moving through the stationary target area may be detected by a significant difference between the signals and the driver alerted before the driver has to make a shoulder check.
This prior invention is most reliable at avoiding false alarms when the fields of view of the two detectors are identical, for example, where the lines of sight of the detectors are parallel. However, parallel lines of sight are not always possible or desirable. Small changes in the mounting can result in misalignments and non-parallel sight lines. The host vehicle itself may exhibit roll, pitch, or other movement during the time between when the readings are taken by the first detector and the second detector, resulting in the two detectors not detecting exactly the same field of view of the target area. The stationary target area may include an abrupt field change, from near to far, such as an end of a guardrail, which can exacerbate differences from non-parallel fields of view, and which can lead to a false alarm. What is desired is an improved method that helps to reduce these false alarm issues for detector systems having two or more detectors.